Previously, libMoltenVK.dylib was created by copying and renaming
the binary from MoltenVK.framework, but this resulted in an unsigned
dylib that was not loadable.
- Create signed macOS libMoltenVK.dylib directly from Xcode.
- Refactor package_moltenvk.sh into package_dylibs.sh & package_headers.sh.
- Remove unused non-packaging Xcode schemes (unrelated).